Thermoplastic material, and finished plastic articles, containing these polychromic substances can be made to change colour upon irradiation with, for example, ultraviolet and/or infra-red radiation.
Compounds based upon diacetylene, X—C≡C—C≡C—Y, when polymerized, are known to take on different colour properties. Polymerisation is typically achieved by exposure to certain types of radiation, such as ultraviolet radiation. Differing the intensity of the radiation causes differing degrees of polymerization, and different colours.
It is known that these properties can be utilized to achieve multi-colour printing. See, for example: U.S. Pat. No. 4,705,742, “Processless multicolour imaging”, issued on Nov. 10th 1987, assigned to Gaf Corporation; and WO2006/018640, “Multi-colour printing”, published on Feb. 23rd 2006, Sherwood Technologies Ltd. Both of these documents disclose methods of applying coatings comprising various diacetylene compounds to the surface of a substrate for the purpose of irradiating and forming an image on the surface of the substrate.
It would, however, be desirable to disperse diacetylene compounds in a thermoplastic material, and then forming finished plastic articles by, for example, blow moulding, injection moulding, extruding. One advantage of achieving such plastic articles would be in the packaging of goods, such as consumer goods, in containers such that any data, colour coding and the like can be inscribed into a bottle or closure late in the packing process, at or even after the packing line. This avoids the need for feeding numerous different bottles and/or closures to a packing line for product change-overs, thus reducing packaging inventory and improving packing efficiency.
However, a problem exists in that diacetylene compounds, such as 10,12-pentacosadiynoic acid, are readily degraded at the processing temperatures which are above the melt temperature of commonly used thermoplastic resins. Such processing temperatures are required for blow moulding, injection moulding, extruding, and the like. In addition, such diacetylene compounds have poor compatibility with, and do not blend into, polyolefins.
The present invention is based upon the finding that certain functional modifications to the diacetylene-based molecule greatly improve its thermal resistance at elevated processing conditions, improve its compatibility with the polymer (e.g. the polyolefin), and improve its compatibility in the finished article. This makes it possible to realize the advantages set out above.